Anyone on the list have experience / knowledge of using PIC microcontrollers
which may be exposed to X-ray beams? A customer has an application for use
in radiation therapy rooms, and the electronics package will occasionally be
in the path of the therapy beam during patient exposure; due to the nature
of the package, this is unavoidable. The question is: will this eventually
cause the electronics - specifically the microcontrollers - to fail? Some
empirical testing done Saturday in a Cancer Center showed that the
microcontrollers will eventually fail, but only after extended exposure -
approx. 1 1/2 hours in a continuous beam, something that will not happen
with a patient. If the package is exposed intermittently, will the
radiation effects be cumulative? Tried Microchip & Google with very little
useful info uncovered. Thanks,
Steve

Thanks David. I found some good information that will be helpful.
Steve

David VanHorn wrote:
> Google on radfet.
>
> It seems that the nature of FETs is to take cumulative damage to the
> gate that shows up as a shift in Vth. This effect can be enhanced,
> but it looks like it's present in all fets to some degree.

> Anyone on the list have experience / knowledge of using PIC microcontrollers
> which may be exposed to X-ray beams? A customer has an application for use
> in radiation therapy rooms, and the electronics package will occasionally be
> in the path of the therapy beam during patient exposure; due to the nature
> of the package, this is unavoidable. The question is: will this eventually
> cause the electronics - specifically the microcontrollers - to fail? Some
> empirical testing done Saturday in a Cancer Center showed that the
> microcontrollers will eventually fail, but only after extended exposure -
> approx. 1 1/2 hours in a continuous beam, something that will not happen
> with a patient. If the package is exposed intermittently, will the
> radiation effects be cumulative? Tried Microchip & Google with very little
> useful info uncovered. Thanks,
> Steve

I would expect any problems to be in program memory retention. Maybe you
should socket the chips and then if replacement or reprogramming turns out
to be required it won't be fatal.

Ionizing radiation will eventually cause threshold shifts and changes in
analog parameters, as well as increases in leakage current. You may find
contemporary information on this sort of thing hard to come by- rad-hard
technology is considered military (or dual-use) by some nations, and
suppling information that can be misused is subject to rather severe
penalties.

>Quoting Steve Moulding <fti1983KILLspamxmission.com>:
>
>> Anyone on the list have experience / knowledge of using PIC microcontrollers
>> which may be exposed to X-ray beams? A customer has an application for use
>> in radiation therapy rooms, and the electronics package will occasionally be
>> in the path of the therapy beam during patient exposure; due to the nature
>> of the package, this is unavoidable. The question is: will this eventually
>> cause the electronics - specifically the microcontrollers - to fail? Some
>> empirical testing done Saturday in a Cancer Center showed that the
>> microcontrollers will eventually fail, but only after extended exposure -
>> approx. 1 1/2 hours in a continuous beam, something that will not happen
>> with a patient. If the package is exposed intermittently, will the
>> radiation effects be cumulative? Tried Microchip & Google with very little
>> useful info uncovered. Thanks,
>> Steve

If things only ahppen after extended exposure, it is reasonable to assume there is a cumulative
effect.
You should certainly implement checks on program memory & eeprom integrity, but the primary defence
should be lead shielding to avoid the exposure in the first place. I'd think that even a relatively
small thickness would provide a substantial improvement over nothing.
You may also want to look at reducing expiosure by suitable orientation - if you can arrange the
electronics to be side-on instead of face-on to the beam, this will reduce the effective dose
substantially.

> Anyone on the list have experience / knowledge of using PIC microcontrollers
> which may be exposed to X-ray beams? A customer has an application for use
> in radiation therapy rooms, and the electronics package will occasionally be
> in the path of the therapy beam during patient exposure; due to the nature
> of the package, this is unavoidable. The question is: will this eventually
> cause the electronics - specifically the microcontrollers - to fail? Some
> empirical testing done Saturday in a Cancer Center showed that the
> microcontrollers will eventually fail, but only after extended exposure -
> approx. 1 1/2 hours in a continuous beam, something that will not happen
> with a patient. If the package is exposed intermittently, will the
> radiation effects be cumulative? Tried Microchip & Google with very little
> useful info uncovered. Thanks,
> Steve
>
>
>
>

Just a single point of data: I recently received a pre-programmed 18F252 by post from the 'States, and I found that all of its memory above a certain point was
erased. It was apparently tested by the sender before despatch and found to be OK, and it re-programmed successfully when I tried that.

The only think I can think would have affected it is being X-Rayed on the journey - being in a package with a lot of electronic parts, I imagine it would have been
"looked" at very closely, so may have had a higher dose than usual. X-Rays would have the same effect as exposing an EPROM to UV - it "leaks" away the charges
that are how the information is stored. As you can't predict which of the program's bytes will be erased first, it could produce some very strange behaviour.

Can you not put some lead over the centre of the package, to protect the chip from X-Rays?

On May 5, 2008, at 1:08 PM, Howard Winter wrote:
> X-Rays would have the same effect as exposing an EPROM to UV - it
> "leaks" away the charges that are how the information is stored.

Except that people who WANTED to use X-rays to erase OTP parts found
that it didn't work. As the argument went, silicon is very
transparent to X-rays... I don't recall ever seeing any actual
quantitative experiments or even deep theory, but there were people
who had actually tried it as an erasure technique and had it not work.

"X-rays will erase the part at high energy levels, but this will
also degrade the part to the point where it will fail soon, or
die during erasure"

and

"This subject has been discussed on sci.electronics multiple
times in the past, and the consensus was that the frequency of
Xrays is such that they have no direct effect on the stored data.
But at very high exposures it is definitely possible to damage
the part"

Note that some coatings will / may cause either scattering
of the X-Rays or emission of secondaries. You may wish to
check with the makers of the machine whether such things
could lead to unintended consequences.

Steve Moulding wrote:
> Anyone on the list have experience / knowledge of using PIC microcontrollers
> which may be exposed to X-ray beams? A customer has an application for use
> in radiation therapy rooms, and the electronics package will occasionally be
> in the path of the therapy beam during patient exposure; due to the nature
> of the package, this is unavoidable. The question is: will this eventually
> cause the electronics - specifically the microcontrollers - to fail? Some
> empirical testing done Saturday in a Cancer Center showed that the
> microcontrollers will eventually fail, but only after extended exposure -
> approx. 1 1/2 hours in a continuous beam, something that will not happen
> with a patient. If the package is exposed intermittently, will the
> radiation effects be cumulative? Tried Microchip & Google with very little
> useful info uncovered. Thanks,
> Steve
>
>
>
>
With sufficient radiation damage, almost all electronic devices will
fail, regardless of source (X-Ray,
Gamma Ray, RF, Microwave).

While x-rays indeed penetrate ICs and other components, lead shielding
also stops the penetration. You
are completely UNABLE to shield your electronics? Lead sheet is very
easy to work, and once shaped
as needed, can be locked into position with plastic sprays. A sheet only
1/8" thick should be sufficient to
reduce the effect to a safe level.

> Anyone on the list have experience / knowledge of using PIC microcontrollers
> which may be exposed to X-ray beams?

Sorry, no first-hand knowledge.

I think the effect will be cumulative, especially on FLASH/OTP program
memory. In addition you should expect (anticipate?) RAM single-cell-flips.

In such a situation I would
- choose the oldest technology (largest chip feature size) I could get
- probably prefer OTP over FLASH
- if at all possible design the code for 'one-shot run' and use an
external 555 or similar to restart the chip for each run

> Anyone on the list have experience / knowledge of using PIC
> microcontrollers
> which may be exposed to X-ray beams? A customer has an application for
> use
> in radiation therapy rooms, and the electronics package will occasionally
> be
> in the path of the therapy beam during patient exposure; due to the nature
> of the package, this is unavoidable. The question is: will this
> eventually
> cause the electronics - specifically the microcontrollers - to fail? Some
> empirical testing done Saturday in a Cancer Center showed that the
> microcontrollers will eventually fail, but only after extended exposure -
> approx. 1 1/2 hours in a continuous beam, something that will not happen
> with a patient. If the package is exposed intermittently, will the
> radiation effects be cumulative? Tried Microchip & Google with very
> little
> useful info uncovered. Thanks,

I remember going to a conference a good many years ago, where one of the
papers described using a microprocessor based system to build a robot that
could be put under an x-ray machine to medical students how to recognise
various items on an x-ray image. He built the dummy with appropriate
internal parts, and had it working nicely.

Then came the day when they tried it under the x-ray. Every thing went
swimmingly for a while, then a limb would give a spasm, and then carry on
normally. Then another would do the same, and so it went as time passed,
until all the limbs were just behaving in a totally random mix of spasms. He
got the unit back to his engineering lab, and found that all the EPROMS had
been erased by the x-rays. The solution was to blow some bipolar PROMS,
which being non-erasable, didn't suffer the same fate.

As to how to deal with your problem, a starting point is to use some 1mm
tantalum sheet top and bottom of the PCB. This is what we do with
instruments on space craft, where chips have doubtful radiation performance,
but then that is shielding from particle damage rather than x-rays. I guess
for x-ray shielding, some similar thickness lead sheet would be the answer.

Steve Moulding wrote:
> Anyone on the list have experience / knowledge of using PIC
> microcontrollers which may be exposed to X-ray beams? ...[snipped]

[PIC] Thank you to everyone who contributed to the discussion.

Maybe as a summary, I can explain what I found out about this issue, partly
from our own testing, and partly from infornation provided by readers of
this list.

X-radiation will affect devices with FET transistors used to implement
logic/memory chip circuitry. Our testing revealed that not only are memory
cells erased, but that on-chip digital control circuitry can be damaged to
the extent that it stops working. Specifically, the Microchip PIC devices
we were testing were found to be readable after exposure, with some (but in
our case not all) memory locations erased. But it was also found that the
chips could not be erased normally and re-programmed - obviously, some other
damage had occurred, rendering the devices unusable.

Also, the effects of radiation are cumulative. Even to the extent that the
effect in FETs is made useful by manufacturing dosimeters that measure the
change in characteristics due to radiation and translating them into
exposure parameters.

Polyethylene can be used as a shield for X-radiation, but the thicknesses
needed for our application would be excessive. Lead shielding is very
effective for shielding purposes, but there is some question about building
the shielding into the product and thereby violating RoHS directives. Since
the device will be used in Europe as well as other parts of the world, this
may be a problem. But there must be some exception for this type of medical
equipment, since lead aprons and other types of personal shielding are still
approved for use in radiation suites / therapy rooms. The required lead
thickness may be prohibitive also - that remains to be seen.

We also discovered that the IR photo IC that we are using to detect the data
stream from the remote control will fail, but at much higher radiation
levels. At this time we have no way of telling whether this is due to FET
failure in the circuitry, of if the detector diode is affected. In any
case, it failed as part of the test.

Rad-hardened devices are available for some functions, but no
microcontrollers as far as I have been able to determine (this may be wrong,
however). It seems that some rad-hardening techniques are classified and so
are not available to the general public. Some are only available for
military and other government entities. And all are prohibitively expensive
for our application, even if available.

Our testing leads us to believe that the inadvertant exposure of our device
to random X-radiation, coupled with the time factor related to exposure will
result in device failures on the order of one or less failures of the
equipment per year. We are considering shielding options, but also making
the affected circuity modular, so that it can be treated as removable /
pluggable by the user should failure occur. That won't make the customer
happy if failure is experienced, but a swap-out program may solve that
issue, since the cost factor is relatively small.

>Polyethylene can be used as a shield for X-radiation, but the thicknesses
>needed for our application would be excessive. Lead shielding is very
>effective for shielding purposes, but there is some question about building
>the shielding into the product and thereby violating RoHS directives. Since
>the device will be used in Europe as well as other parts of the world, this
>may be a problem. But there must be some exception for this type of medical
>equipment, since lead aprons and other types of personal shielding are still
>approved for use in radiation suites / therapy rooms. The required lead
>thickness may be prohibitive also - that remains to be seen.

Have you considered other materials? I've used tantalum and tungsten
in the past. Neither is as inexpensive as lead, unfortunately, and
machining the latter is a PITA. What is the maximum X-ray energy (kV
on the tube)?

>> Polyethylene can be used as a shield for X-radiation, but the
>> thicknesses needed for our application would be excessive. Lead
>> shielding is very effective for shielding purposes, but there is
>> some question about building the shielding into the product and
>> thereby violating RoHS directives. Since the device will be used in
>> Europe as well as other parts of the world, this may be a problem.
>> But there must be some exception for this type of medical equipment,
>> since lead aprons and other types of personal shielding are still
>> approved for use in radiation suites / therapy rooms. The required
>> lead thickness may be prohibitive also - that remains to be seen.
>
> Have you considered other materials? I've used tantalum and tungsten
> in the past. Neither is as inexpensive as lead, unfortunately, and
> machining the latter is a PITA. What is the maximum X-ray energy (kV
> on the tube)?
>
> --
>
> ---
> Chris Smolinski
> Black Cat Systems
> http://www.blackcatsystems.com

Hi Chris,
Tantalum might be a possibility, especially as it is available in many
forms. The machine we are dealing with is a therapy machine generating
X-rays at 6 MV.
Steve

> Chris Smolinski wrote:
>>> Polyethylene can be used as a shield for X-radiation, but the
>>> thicknesses needed for our application would be excessive. Lead
>>> shielding is very effective for shielding purposes, but there is
>>> some question about building the shielding into the product and
>>> thereby violating RoHS directives. Since the device will be used in
>>> Europe as well as other parts of the world, this may be a problem.
>>> But there must be some exception for this type of medical equipment,
>>> since lead aprons and other types of personal shielding are still
>>> approved for use in radiation suites / therapy rooms. The required
>>> lead thickness may be prohibitive also - that remains to be seen.
>>
>> Have you considered other materials? I've used tantalum and tungsten
>> in the past. Neither is as inexpensive as lead, unfortunately, and
>> machining the latter is a PITA. What is the maximum X-ray energy (kV
>> on the tube)?
>>
>> --
>>
>> ---
>> Chris Smolinski
>> Black Cat Systems
>> http://www.blackcatsystems.com
>
> Hi Chris,
> Tantalum might be a possibility, especially as it is available in many
> forms. The machine we are dealing with is a therapy machine
> generating X-rays at 6 MV.
> Steve

It just occured to me that maybe I haven't made it clear that this is not a
regular x-ray machine as most people think of them. The device is a linear
accelerator, which accelerates electrons to the desired energy, smashes them
into a metallic target generating high-energy x-rays, and then collects,
focuses,and collimates the x-rays into a beam that is used to radiate
cancerous tumors accurately. The procedure is usually referred to as
external beam therapy. The beam that is generated is the one that will
subject our electronics to x-radiation after having passed through the
patient.

Try this url for an example (sorry about the length - I couldn't get TinyURL
to work for me).

> > Hi Chris,
>> Tantalum might be a possibility, especially as it is available in many
>> forms. The machine we are dealing with is a therapy machine
>> generating X-rays at 6 MV.
>> Steve

Ahh, 6 MeV. Yes, you'll need a lot of lead/tungsten/etc. The mfp of
Pb at 6 MeV is around 6 cm, for W closer to 3.5. How about using DU
for shielding? That should give the RoHS goons something to worry
about. Hey, at least it ain't lead. (The mfp at 6 MeV is 2.5 cm so it
is slightly better)

>It just occured to me that maybe I haven't made it clear that this is not a
>regular x-ray machine as most people think of them. The device is a linear
>accelerator, which accelerates electrons to the desired energy, smashes them
>into a metallic target generating high-energy x-rays, and then collects,
>focuses,and collimates the x-rays into a beam that is used to radiate
>cancerous tumors accurately. The procedure is usually referred to as
>external beam therapy. The beam that is generated is the one that will
>subject our electronics to x-radiation after having passed through the
>patient.

Of course the vast majority of the x-rays are at lower energies, with
peaks at the characteristic energies for the target material. In
fact, chances are that most of the damage is being done by lower
energy x-rays anyway, as the higher energies are zipping through the
Si without causing damage. Well, most of them. You might find that
you only need to shield up to a few hundred kV? Insert hand waving
here. Or are the lower energies already being attenuated by the
machine/patient?

>We also discovered that the IR photo IC that we are using to detect the
>data stream from the remote control will fail, but at much higher radiation
>levels. At this time we have no way of telling whether this is due to FET
>failure in the circuitry, of if the detector diode is affected. In any
>case, it failed as part of the test.

LEDs and photodiodes also suffer significant degradation under radiation.
Using opto-isolators on spacecraft is a necessary evil that requires a fair
amount of thinking about degraded performance of opto devices.